User:Danstan

1.0. INTRODUCTION
Ground improvement techniques vary on a site-by-site basis. Differences in soil characteristics influence the method to be employed. Factors that affect the chosen method include soil type, water table depth, soil grain size, depth to soils of concern, overlying soil characteristics, potential for seismic activity, site location, and future site use requirements. Dynamic compaction is a method that is used to increase the density of the soil deposited both above and below the ground water table, when certain subsurface constraints make other methods inappropriate. The process involves of dropping a heavy weight repeatedly on the ground at regularly spaced intervals. The weight and the height determine the amount of compaction that would occur. The weight that is used depends on the degree of compaction desired and is between 10 to 20 tons up to 150 tons. The height varies from 10 to 20m up to 40m. High technical knowledge and experience is required to densify the ground of various characteristics below the water table uniformly to a desired density. Unless properly guided with technical management, simple tamping with a heavy hammer may result in non-uniform ground densified only in a few meters below the ground surface or other defects in the ground treated by tamping. Properly planned tamping work based on technical management is therefore important for the success of the Dynamic Compaction method.

2.0. ADVANTAGES OF DYNAMIC COMPACTION
This method is adopted due to following merits.

	Water is not necessary for this method. In this method no need water to continue the process like in other methods.

	The method is relatively simple. No need much equipment for the process.

	The method is relatively flexible. The pounder weight, height of drop, number of impacts per site, and spacing can all be varied to optimize the operation.

	This technique is appropriate for application to non cohesive soils and fills and serves to improve the bearing capacity and settlement characteristics of the ground for the foundations of buildings and in civil applications, embankments and where loose soils can be improved to render them serviceable.

	Dynamic Compaction provides a very economical ground improvement method, where its application is appropriate.

3.0. SELECT PROPER GROUND IMPROVEMENT TECHNIQUE
Prior to start a project, It should be implemented a site investigation survey with required number of bore holes that are investigated up to hard soil strata. Bearing capacity of soil, cohesion, friction angle etc can be found by site investigation report. Also, the important parameters for the soil that affect the suitability of dynamic compaction include the soil classification, Groundwater level, Relative density, degree of saturation and permeability plus length of drainage paths. Following table indicates the suitability of various deposits for dynamic compaction based upon those factors. We can select conclude that Dynamic compaction method is good or not according to soil classification from this table. General soil type	Potential suitability for Dynamic Compaction technique Well graded water pervious deposits such as • Building Rubble • Boulders • Broken Concrete	Excellent Pervious water deposits containing not more than 35% silt • Decomposed landfills	Good

Excellent if material has a low water saturation

Semi-pervious soil deposits With less than 25% clay	Not recommended

If low water saturation in material minor compaction improvements can be made Miscellaneous fill • Recent Municipal landfill	Not recommended

Long term settlement will occur due to decomposition, could be used for embankments Highly Organic Deposits • Peat organic silts 	Not recommended

Unless sufficient granular fill can be added and mixed with the organic deposits Suitability of Deposits for Dynamic Compaction Most natural soil or fill deposits can be characterized by index tests including grain size gradation and/or Atterberg limits. On a typical gradation chart, three zones are shown in following chart. Zone 1 represents the gradation range where dynamic compaction is most appropriate. Zone 3 is the gradation of fully or nearly saturated. Zone 2 is the transition range where dynamic compaction will work but multiple passes are required to allow excess pore pressures to dissipate before more energy is applied. Grouping of Deposits for Dynamic Compaction From above table or chart we can take decision that dynamic compaction is more appropriate for this site or not. If dynamic compaction method is suitable, we have to design required parameters. Otherwise, another method should be used.

4.0. DESIGN OF DYNAMIC COMPACTION
The design of dynamic compaction is defined to assess various parameters of the method and specify the points of engineering management during construction. Tamping energy per blow is given by D = n(WH)0.5 D = Depth of improvements in meters. This depth can select using site investigation data. W = Weight of hammer in tons. H = Drop height in meters n = A coefficient. We can design values for W and H according to the D value that we need to improve the soil up to required depth.

Soil type	Degree of Saturation	Recommended ‘n’ value Pervious soil deposit granular	High Low	0.5 0.5 to 0.6 Semi pervious soil deposits, Primary silt with PI<8	High Low	0.35 to 0.4 0.4 to 0.5 Impervious deposits, primarily clay soils with PI>8	High Low	Not recommended 0.35 to 0.4 Recommended n values for different soil types In dynamic compaction work, a site is marked off in a grid pattern and one tamping point located in each grid. The grid dimensions have to be selected to densify the ground uniformly. Tamping at too short distance internally densifies only a shallow layer near the ground surface in the early stage of tamping work and the compacted shallow layer will interrupt propagation of the tamping energy generated by subsequent passes. As a common practice a larger dimension is selected in the first pass and in the next pass tamping points are selected between the imprints in the first pass. As initial estimate the grid spacing for the first pass can be taken equal to the desired depth of treatment. Excess pore pressure develops in below the ground water table when the ground is subjected to tamping. Part of shock pore pressure developed by the impacts of hammer remains. This residual pore pressure is accumulated and increases with the progress of tamping work. The excess pore water pressure developed by tamping varies widely depending on density and permeability of the ground. The developed pore pressure is large in loose soil and small in dense soil. In the ground of low permeability, accumulated excess pore water pressure remains and increases during tamping. In very pervious ground however, pore pressure caused by a blow of a hammer dissipates entirely before the next and sometimes no residual pore pressure is found.

5.0. METHODOLOGY
The area to be treated is divided into grid patterns with each grid point receiving several blows in a given pass. Several passes may be necessary to obtain the desired results. It is common to conceptualize the compaction treatment as a series of compaction phases with different combination of energy levels designed to achieve improvement to specific depth. The first phase is generally aimed at compacting the deepest layer by adapting a relatively wide grid pattern and a suitable number of blows. The next phase will involve compacting the middle layer. This is often carried out at the mid-point of the first phase with lesser number of blows and a reduced drop height. The upper surface layer will then be compacted on a continuous overlapping of compaction points from a lower energy blows. It is very often that the upper 0.5 - 1m will not be compacted sufficiently and it is normally roller compacted to finish the compaction works. The construction sequence, 	Excavation of upper 2.5m recent weak deposits 	Backfill with 2.5m of clean sand as working platform and also as drainage blanket. 	Performed dynamic replacement on structural areas and dynamic compaction over the entire treatment area (Phase 1). 	Performed phase 2 and ironing phase of ground improvement work. 	Carry out quality control, instrumentation and monitoring works during and after ground improvement works. 	Complete sandfilling to reach finished platform level with compaction to 90% modified proctor standard. 	Carried out 2m surcharging for 6 weeks. Settlement monitoring. 	Surcharge removed and proceeds with construction.